U.S. patent number 7,005,674 [Application Number 10/769,816] was granted by the patent office on 2006-02-28 for organic thin film transistor comprising multi-layered gate insulator.
This patent grant is currently assigned to Samsung Electronics Co., Ltd.. Invention is credited to Young Hun Byun, In Nam Kang, Bon Won Koo, Sang Yoon Lee, Yi Yeol Lyu, Jong Jin Park.
United States Patent |
7,005,674 |
Lee , et al. |
February 28, 2006 |
Organic thin film transistor comprising multi-layered gate
insulator
Abstract
An organic thin film transistor (OTFT) comprising a gate
electrode, a gate insulating film, an organic active layer and a
source/drain electrode, or a gate electrode, a gate insulating
film, a source/drain electrode and an organic active layer,
sequentially formed on a substrate, wherein the gate insulating
film is a multi-layered insulator comprising a first layer of a
high dielectric material and a second layer of an insulating
organic polymer compatible with the organic active layer, the
second layer being positioned directly under the organic active
layer. The OTFT of the present invention shows low threshold and
driving voltages, high charge mobility, and high
I.sub.on/I.sub.off, and it can be prepared by a wet process.
Inventors: |
Lee; Sang Yoon (Seoul,
KR), Park; Jong Jin (Gyeonggi-Do, KR), Lyu;
Yi Yeol (Daejeon-Si, KR), Byun; Young Hun
(Daejeon-Si, JP), Koo; Bon Won (Gyeonggi-Do,
KR), Kang; In Nam (Gyeonggi-Do, KR) |
Assignee: |
Samsung Electronics Co., Ltd.
(Suwon-si, KR)
|
Family
ID: |
33432455 |
Appl.
No.: |
10/769,816 |
Filed: |
February 3, 2004 |
Prior Publication Data
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Document
Identifier |
Publication Date |
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US 20050001210 A1 |
Jan 6, 2005 |
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Foreign Application Priority Data
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Jul 3, 2003 [KR] |
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10-2003-0044799 |
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Current U.S.
Class: |
257/40; 438/287;
438/99; 257/411; 257/E21.274; 257/E21.272 |
Current CPC
Class: |
H01L
51/0529 (20130101); H01L 51/0545 (20130101); H01L
21/31604 (20130101); H01L 21/02197 (20130101); H01L
51/0052 (20130101); H01L 51/0537 (20130101); H01L
21/02282 (20130101); H01L 21/02183 (20130101); H01L
21/02175 (20130101); H01L 21/02194 (20130101); H01L
21/02186 (20130101); H01L 21/02189 (20130101); H01L
21/02118 (20130101); H01L 21/02205 (20130101); H01L
21/31691 (20130101) |
Current International
Class: |
H01L
35/24 (20060101); H01L 29/76 (20060101) |
Field of
Search: |
;257/40,288,289,411
;438/99,151,158,287 |
References Cited
[Referenced By]
U.S. Patent Documents
Other References
C D. Dimitrakopoulos et al.; Low-Voltage Organic Transistors on
Plastic Comprising High-Dielectric Constant Gate Insulators;
Report; Feb. 5, 1999; pp. 822-824; vol. 283; Science, USA. cited by
other .
Y. Y. Lin et al.; High-Mobility Pentacene Organic Thin Film
Transistors; 54.sup.th Annual Device Research Conference Digest;
1996; pp. 80-81. cited by other.
|
Primary Examiner: Nadav; Ori
Attorney, Agent or Firm: Buchanan Ingersoll PC
Claims
What is claimed is:
1. An organic thin film transistor, comprising a gate electrode, a
gate insulating film, an organic active layer and a source/drain
electrode, or a gate electrode, a gate insulating film, a
source/drain electrode and an organic active layer, sequentially
located on a substrate, wherein the gate insulating film is a
multi-layered insulator comprising a first layer of a high k
material and a second layer of an insulating organic polymer
compatible with the organic active layer, the second layer being
positioned directly under the organic active layer, wherein the
insulating organic polymer of the second insulating layer is
selected from the group consisting of polyvinylphenol,
polyacrylate, polyvinylalcohol, and a polymer represented by the
following Formula 1: ##STR00008## wherein, R is represented by the
following Formula 2: ##STR00009## wherein R.sub.1 is selected from
the group consisting of the following groups of group A, in which n
is an integer of 0 to 10: ##STR00010## R.sub.2 is a photo-alignment
group selected from the group consisting of the following groups of
Group B, provided that at least one of R.sub.2 is selected from (I)
when I is 2 or higher: ##STR00011## ##STR00012## R.sub.3 is a
hydrogen atom or is selected from the group consisting of the
following groups of Group C, in which X is a hydrogen atom, an
alkyl or alkoxy group of 1 to 13 carbon atoms, an aromatic group of
6 to 20 carbon atoms, a hetero-aromatic group of 4 to 14 carbon
atoms having at least one hetero atom contained in an aromatic
ring, --(OCH.sub.2).sub.pCH.sub.3 wherein p is an integer of 0 to
12, F or Cl and m is an integer of 0 to 18: ##STR00013## k is an
integer of 0 to 3 and I is an integer of 1 to 5, provided that each
of R.sub.1 and R.sub.2 can be different when k or I is 2 or higher;
m is a real number of 0.3 to 0.7, and n is a real number of 0.3 to
0.7, provided that the sum of m and n becomes 1; x is a real number
of 0.3 to 0.7, and y is a real number of 0.3 to 0.7, provided that
the sum of x and y becomes 1; and i is a real number of 0 to 1 and
j is a real number of 0 to 1, provided that the sum of i and j
becomes 1.
2. The organic thin film transistor of claim 1, wherein the polymer
represented by the Formula 1 is a compound represented by the
following Formulas 3, 4, 5, or 6: ##STR00014## ##STR00015##
##STR00016##
3. The organic thin film transistor of claim 1, wherein the first
and the second layers of the gate insulating film are formed by a
wet process.
4. The organic thin film transistor of claim 1, wherein the
substrate is plastic, glass, quartz, or silicon substrate.
5. The organic thin film transistor of claim 3, wherein the wet
process is carried out by a spin coating, a dip coating, a
printing, or a roll coating method.
6. The organic thin film transistor of claim 1, wherein the organic
active layer is made of any one selected from the group consisting
of pentacene, copper phthalocyanine, polythiophene, polyaniline,
polyacetylene, polypyrrole, polyphenylene vinylene, and derivatives
thereof.
7. The organic thin film transistor of claim 1, wherein the high k
material for the first insulating layer is a mixture of an
insulating organic polymer and an organic metal compound, or a
mixture of an insulating organic polymer and nanoparticles of an
inorganic metal oxide or a ferroelectric insulator, wherein the
high k material has a dielectric constant (k) of 5 or higher.
8. The organic thin film transistor of claim 7, wherein the
insulating organic polymer for the first layer is selected from the
group consisting of polyester, polycarbonate, polyvinylalcohol,
polyvinylbutyral, polyacetal, polyarylate, polyamide,
polyamidimide, polyetherimide, polyphenylenether,
polyphenylenesulfide, polyethersulfone, polyetherketone,
polyphthalamide, polyethernitrile, polyethersulfone,
polybenzimidazole, polycarbodiimide, polysiloxane,
polymethylmethacrylate, polymethacrylamide, nitrile rubbers, acryl
rubbers, polyethylenetetrafluoride, epoxy resins, phenol resins,
melamine resins, urea resins, polybutene, polypentene,
ethylene-co-propylene, ethylene-co-butene diene, polybutadiene,
polyisoprene, ethylene-co-propylene diene, butyl rubbers,
polymethylpentene, polystyrene, styrene-co-butadiene, hydrogenated
styrene-co-butadiene, hydrogenated polyisoprene, hydrogenated
polybutadiene, and mixtures thereof.
9. The organic thin film transistor as defined in claim 7, wherein
the organic metal compound for the first layer is selected from the
group consisting of titanium-based compounds, including titanium
(IV) n-butoxide, titanium (IV) t-butoxide, titanium (IV) ethoxide,
titanium (IV) 2-ethylhexoxide, titanium (IV) isopropoxide, titanium
(IV) (di-isopropoxide)bis-(acetylacetonate), titanium (IV) oxide
bis(acetylacetonate), trichlorotris(tetrahydrofuran)titanium (III),
tris(2,2,6,6-tetramethyl-3,5-heptanedionato)titanium (III),
(trimethyl)pentamethyl cyclopentadienyl titanium (IV),
pentamethylcyclopentadienyltitanium trichloride (IV),
pentamethylcyclopentadienyltitanium trimethoxide (IV),
tetrachlorobis(cyclohexylmercapto)titanium (IV),
tetrachlorobis(tetrahydrofuran)titanium (IV),
tetrachlorodiamminetitanium (IV), tetrakis(diethylamino)titanium
(IV), tetrakis(dimethylamino)titanium (IV),
bis(t-butylcyclopentadienyl)titanium dichloride,
bis(cyclopentadienyl)dicarbonyl titanium (II),
bis(cyclopentadienyl)titanium dichloride,
bis(ethylcyclopentadienyl)titanium dichloride,
bis(pentamethylcyclopentadienyl)titanium dichloride,
bis(isopropylcyclopentadienyl)titanium dichloride,
tris(2,2,6,6-tetramethyl-3,5-heptanedionato)oxotitanium (IV),
chlorotitanium triisopropoxide, cyclopentadienyltitanium
trichloride, dichlorobis(2,2,6,6-tetramethyl-3,5-heptane dionato)
titanium (IV), dimethylbis(t-butylcyclopentadienyl)titanium (IV),
or di(isopropoxide)bis
(2,2,6,6-tetramethyl-3,5-heptanedionato)titanium (IV); zirconium-
or hafnium-based compounds, including zirconium (IV) n-butoxide,
zirconium (IV) t-butoxide, zirconium (IV) ethoxide, zirconium (IV)
isopropoxide, zirconium (IV) n-propoxide, zirconium (IV)
acetylacetonate, zirconium (IV) hexafluoroacetylacetonate,
zirconium (IV) trifluoroacetylacetonate,
tetrakis(diethylamino)zirconium, tetrakis(dimethylamino)zirconium,
tetrakis(2,2,6,6-tetramethyl-3,5-heptanedionato)zirconium (IV),
zirconium (IV) sulfate tetrahydrate, hafnium (IV) n-butoxide,
hafnium (IV) t-butoxide, hafnium (IV) ethoxide, hafnium (IV)
isopropoxide, hafnium (IV) isopropoxide monoisopropylate, hafnium
(IV) acetylacetonate, or tetrakis(dimethylamino)hafnium;
aluminum-based compounds, including aluminum n-butoxide, aluminum
t-butoxide, aluminum s-butoxide, aluminum ethoxide, aluminum
isopropoxide, aluminum acetylacetonate, aluminum
hexafluoroacetylacetonate, aluminum trifluoroacetylacetonate, or
tris(2,2,6,6-tetramethyl-3,5-heptanedionato) aluminum; and mixtures
thereof.
10. The organic thin film transistor as defined in claim 7, wherein
the nanoparticles of the inorganic metal oxide comprise
nanoparticles of Ta.sub.2O.sub.5, Y.sub.2O.sub.3, TiO.sub.2,
CeO.sub.2, or ZrO.sub.2, and the nanoparticles of the ferroelectric
insulator comprise nanoparticles of barium strontium titanate
(BST), PbZr.sub.xTi.sub.1-xO.sub.3 (PZT), Bi.sub.4Ti.sub.3O.sub.12,
BaMgF.sub.4, SrBi.sub.2(Ta.sub.1-xNb.sub.x).sub.2O.sub.9,
Ba(Zr.sub.1-xTi.sub.x)O.sub.3 (BZT), BaTiO.sub.3, SrTiO.sub.3 or
Bi.sub.4Ti.sub.3O.sub.12, in which the nanoparticles have diameters
of 1 100 nm.
Description
This non-provisional application claims priority under 35 U.S.C.
.sctn.119(a) from Korean Patent Application No. 2003-44799 filed on
Jul. 3, 2003, which is herein incorporated by reference.
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to organic thin film transistors, and
in particular, to an organic thin film transistor including a gate
electrode, a gate insulating film, an organic active layer and a
source/drain electrode, or a gate electrode, a gate insulating
film, a source/drain electrode and an organic active layer,
sequentially formed on a substrate, wherein the gate insulating
film is a multi-layered insulator comprising a first layer of a
high dielectric constant (k) material and a second layer of an
insulating organic polymer compatible with the organic active
layer, the second layer being positioned directly under the organic
active layer.
2. Description of the Related Art
In recent years, most of thin film transistors (TFT) used for
display application consisted of amorphous silicon as the
semiconductor, silicon oxide, or silicon nitride as the insulator,
and metal electrodes. However, with the recent development of
various conductive organic materials, research into developing an
organic thin film transistor (OTFT) using an organic material as
the semiconductor has been made actively. Since its first
development in the 1980s, the OTFT has widened its application into
functional electronic devices and optical devices. For example, in
the field of liquid crystal displays (LCD), which includes the TFT
as switching elements controlling the electric fields, there are
many attempts to adopt the OTFT due to its flexibility and easy
preparing process. As novel electronic material, the organic
semiconductor in the OTFT is superior to its inorganic counterpart
(i.e. amorphous silicon) because it has many synthetic routes and
can be formed in any shape from fiber to film. Further it shows
high flexibility and can be manufactured at a low cost. Therefore,
the OTFT using the organic semiconductor such as conducting
polymers as an active layer is considered to be advantageous in
that the overall manufacture can be achieved by a roll to roll
process using a plastic substrate because its active layer can be
formed by a printing-process under atmospheric pressure, instead of
chemical vapor deposition (CVD) using plasma and requiring high
pressure and high temperature, so low-priced TFT could be
realized.
But, compared with the amorphous Si TFT, the OTFT exhibits
disadvantageously lower charge mobility and higher driving and
threshold voltages. In this regard, N. Jackson et al. made an
improvement and raised possibility for the OTFT's practical use by
achieving a charge mobility of 0.6 cm.sup.2V.sup.-1sec.sup.-1 with
pentacene active layer (54.sup.th Annual device Research Conference
Digest 1996). However, the charge mobility achieved by N. Jackson
still falls short of the required value, and as well, the OTFT in
the prior art requires a driving voltage higher than 100 V and a
sub-threshold voltage at least 50 times as high as that of
amorphous silicon-TFT. Meanwhile, in U.S. Pat. No. 5,981,970 and
Science (Vol. 283, pp822 824), there is disclosed a method of
lowering the driving voltage and the threshold voltage in the OTFT
by use of a high dielectric constant (i.e. high k) gate insulator,
in which the gate insulator is made of an inorganic metal oxide
such as Ba.sub.xSr.sub.1-xTiO.sub.3 (BST; Barium Strontium
Titanate), Ta.sub.2O.sub.5, Y.sub.2O.sub.3, and TiO.sub.2, or a
ferroelectric insulator such as PbZr.sub.xTi.sub.1-xO.sub.3(PZT),
Bi.sub.4Ti.sub.3O.sub.12, BaMgF.sub.4,
SrBi.sub.2(Ta.sub.1-xNb.sub.x).sub.2O.sub.9,
Ba(Zr.sub.1-xTi.sub.x)O.sub.3 (BZT), BaTiO.sub.3, SrTiO.sub.3, and
Bi.sub.4Ti.sub.3O.sub.12. In the OTFT prepared by said method, the
gate insulator was prepared by chemical vapor deposition, physical
vapor deposition, sputtering, or sol-gel coating techniques and its
dielectric constant, k, was 15 or higher. By using this high k
insulator, the driving voltage can be decreased to -5V, but the
charge mobility still remains unsatisfactory, lower than 0.6
cm.sup.2V.sup.-1sec.sup.-1. Further, since the process requires
high temperatures of 200 400.degree. C., there is a limit in
selecting the type of the substrate and as well, it becomes
impossible to adopt a common wet process such as simple coating or
printing. U.S. Pat. No. 6,232,157 discloses a method of using
polyimide, benzocyclobutene or polyacryl as the organic insulating
film, but, the OTFT prepared by the method cannot exhibit device
characteristics equal to those of the TFT of inorganic
insulator.
In order to improve the performance of thin film electronic devices
in the prior art, there were many attempts to adopt a multi-layered
gate insulator having two or more layers. For example, U.S. Pat.
Nos. 6,563,174 and U.S. Pat. No. 6,558,987 disclose a multi-layered
gate insulating film made of amorphous silicon nitride and silicon
oxide and a double insulating film made of the same material,
respectively, and both of the patents reported that there was a
substantial improvement in electrical property of the insulator and
crystalline quality of the semiconductor layer. However, these
patents are inherently related to the inorganic TFT using the
inorganic material, such as amorphous or monocrystalline silicon,
and thus cannot be applied in the preparation of the organic
semiconductor device.
Recently, many attempts have been made to use the OTFT for various
drive devices. However, to realize the practical use of OTFT in LCD
or flexible displays using organic EL, not only should a charge
mobility increase to the level of 5 cm.sup.2V.sup.-1sec.sup.-1 or
higher, but also improvement in the driving and threshold voltages
of the device should be achieved. In particular, for simplifying
the preparation and reducing the cost, it can be desirable for the
whole process of preparing the OTFT to be carried out by an
all-printing or all-spin method on a plastic substrate. Under the
circumstances, there have been many research efforts for developing
a method to simplify the preparation of the organic gate insulating
film and to increase the charge mobility in the interface between
the insulator and the organic active layer. However, satisfactory
results have yet to be obtained.
Thus, in this art, it is urgently demanded to develop an organic
TFT of a new structure that shows high charge mobility, superior
insulating properties, and low driving and threshold voltages, and
that can be prepared with ease, for example, by a common wet
process.
SUMMARY OF THE INVENTION
The present inventors devoted much effort to meet these demands and
found that, when using a multi-layered gate insulator including a
first layer of a high k material and a second layer of an
insulating polymer being compatible with an organic active layer
and positioned directly beneath the organic active layer, the OTFT
thus obtained exhibits a higher charge mobility and a lower driving
and threshold voltages and its whole preparation can be achieved by
a wet process, such as printing or spin coating.
Therefore, a feature of an embodiment of the present invention is
to provide an organic thin film transistor comprising a gate
electrode, a gate insulating film, an organic active layer and a
source/drain electrode, or a gate electrode, a gate insulating
film, a source/drain electrode and an organic active layer,
sequentially formed on a substrate, wherein the gate insulating
film is a multi-layered insulator comprising i) a first layer of a
high dielectric material and ii) a second layer of an insulating
organic polymer being compatible with the organic active layer and
positioned directly under the organic active layer.
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 is a schematic cross-sectional view of an organic thin film
transistor (OTFT) according to a preferable embodiment of the
present invention;
FIG. 2 is a plot showing the change of leakage current as varying
the applied voltage to the OTFT's prepared in Examples 1 and 3 and
Comparative Example 1;
In FIG. 3 are shown the measured operating characteristics of the
OTFTs prepared in Example 1 and Comparative Example 1 respectively,
by a plot of the drain current as a function of the gate voltage;
and
In FIG. 4 are shown plots of the square root of the drain current
as a function of gate voltage of prepared in Example 1 and
Comparative Example 1 and, from the plots, threshold voltages of
the OFTF can be obtained.
DETAILED DESCRIPTION OF THE INVENTION
As shown in FIG. 1, an OTFT according to the present invention
includes (a) a substrate 1 on which one or more gate electrode(s)
is (are) disposed, (b) one or more gate electrode(s) 5, (c) a
multi-layered gate insulating film disposed on the gate
electrode(s) 5 comprising (i) a first insulating layer 2 of high-k
material and (ii) a second insulating layer 3 of an organic polymer
being compatible with and positioned directly beneath an organic
active layer 4, (d) an organic semiconductor layer as the organic
active layer 4 disposed on the gate insulating film, and (e) a
source/drain electrode (6 and 7).
As mentioned above, the layering order between the organic active
layer and the source/drain electrode may be changed relative to
each other.
FIG. 1 shows schematically one preferable embodiment of the present
invention so the gate insulating film of the OTFT of the present
invention, consisting of two layers in FIG. 1, may have two or more
layer as required. A total effective dielectric constant of the
gate insulating film can be adjusted by controlling the thickness
of the first insulating layer 2 and second insulating layer 3.
In the present invention, the first layer of the gate insulating
film is composed of a high k material having both high dielectric
constant (k) and excellent insulating properties, and it is formed
by a wet process. Specifically, the first insulating layer 2 is
made of (1) a mixture of an insulating organic polymer and an
organic metal compound having a dielectric constant of 5 or higher,
or (2) a mixture of an insulating organic polymer and nanoparticles
of an inorganic metal oxide or ferroelectric insulator having a
dielectric constant of 5 or more. The dielectric constant `k` of
the first layer can be adjusted by controlling a weight ratio
between the organic polymer and the organic metal compound or the
nanoparticles. The dielectric constant of the first insulating
layer should be controlled at 5 or higher and, in the case of the
dielectric constant being less than 5, an improvement of drive
properties is more difficult to achieve due to the lower effective
dielectric constant. For formation of the first layer, the mixture
is coated on the substrate including the gate electrode by the wet
process, and then baked.
The insulating organic polymer useful for preparing the first
insulating layer includes most polymers exhibiting insulating
properties. The examples of the insulating organic polymer include,
but are not limited to, polyester, polycarbonate, polyvinylalcohol,
polyvinylbutyral, polyacetal, polyarylate, polyamide,
polyamidimide, polyetherimide, polyphenylenether,
polyphenylenesulfide, polyethersulfone, polyetherketone,
polyphthalamide, polyethernitrile, polyethersulfone,
polybenzimidazole, polycarbodiimide, polysiloxane,
polymethylmethacrylate, polymethacrylamide, nitrile rubbers, acryl
rubbers, polyethylenetetrafluoride, epoxy resins, phenol resins,
melamine resins, urea resins, polybutene, polypentene,
poly(ethylene-co-propylene), poly(ethylene-co-butenediene),
polybutadiene, polyisoprene, poly(ethylene-co-propylene diene),
butyl rubbers, polymethylpentene, polystyrene,
poly(styrene-co-butadiene), hydrogenated
poly(styrene-co-butadiene), hydrogenated polyisoprene, hydrogenated
polybutadiene and mixtures thereof.
The organic metal compound used for the first insulating layer is
titanium-, zirconium-, hafnium- and aluminum-based organic metal
compounds. Examples of the titanium-based compounds include, but
are not limited to, titanium (IV) n-butoxide, titanium (IV)
t-butoxide, titanium (IV) ethoxide, titanium (IV) 2-ethylhexoxide,
titanium (IV) isopropoxide, titanium (IV)
(di-isopropoxide)bis-(acetylacetonate), titanium (IV) oxide
bis(acetylacetonate), trichlorotris(tetrahydrofuran)titanium (III),
tris(2,2,6,6-tetramethyl-3,5-heptanedionato)titanium(III),
(trimethyl)pentamethyl cyclopentadienyl titanium (IV),
pentamethylcyclopentadienyltitanium trichloride (IV),
pentamethylcyclo-pentadienyltitanium trimethoxide (IV),
tetrachlorobis(cyclohexylmercapto)titanium (IV),
tetrachlorobis(tetrahydrofuran)titanium (IV),
tetrachlorodiamminetitanium (IV), tetrakis(diethylamino)titanium
(IV), tetrakis(dimethylamino)titanium (IV),
bis(t-butylcyclopentadienyl)titaniumdichloride, bis(cyclopenta
dienyl)dicarbonyltitanium (II), bis(cyclopentadienyl)titanium
dichloride, bis(ethylcyclopentadienyl)titanium dichloride,
bis(pentamethylcyclopentadienyl)titanium dichloride,
bis(isopropylcyclopentadienyl)titanium dichloride,
tris(2,2,6,6-tetramethyl-3,5-heptanedionato)oxotitanium (IV),
chlorotitanium triisopropoxide, cyclopentadienyltitanium
trichloride, dichlorobis(2,2,6,6-tetramethyl-3,5-heptane
dionato)titanium (IV), dimethylbis(t-butylcyclopentadienyl)titanium
(IV) and di(isopropoxide)bis
(2,2,6,6-tetramethyl-3,5-heptanedionato)titanium (IV). Examples of
the zirconium-based compounds include, but are not limited to,
zirconium (IV) n-butoxide, zirconium (IV) t-butoxide, zirconium
(IV) ethoxide, zirconium (IV) isopropoxide,
zirconium(IV)n-propoxide, zirconium (IV) acetylacetonate, zirconium
(IV) hexafluoroacetylacetonate, zirconium (IV)
trifluoroacetylacetonate, tetrakis(diethylamino)zirconium,
tetrakis(dimethylamino)zirconium,
tetrakis(2,2,6,6-tetramethyl-3,5-heptanedionato)zirconium (IV) and
zirconium (IV) sulfate tetrahydrate. Examples of the hafnium-based
compounds include, but are not limited to, hafnium (IV) n-butoxide,
hafnium (IV) t-butoxide, hafnium (IV) ethoxide, hafnium (IV)
isopropoxide, hafnium (IV) isopropoxide monoisopropylate, hafnium
(IV) acetylacetonate and tetrakis(dimethylamino)hafnium. Examples
of the aluminum-based compounds include, but are not limited to,
aluminum n-butoxide, aluminum t-butoxide, aluminum s-butoxide,
aluminum ethoxide, aluminum isopropoxide, aluminum acetylacetonate,
aluminum hexafluoroacetylacetonate, aluminum
trifluoroacetylacetonate and
tris(2,2,6,6-tetramethyl-3,5-heptanedionato) aluminum.
The nanoparticles of the metal oxide used for the preparation of
the first insulating layer include, but are not limited to,
nanoparticles of Ta.sub.2O.sub.5, Y.sub.2O.sub.3, TiO.sub.2,
CeO.sub.2, and ZrO.sub.2. The nanoparticles of the metal oxide have
preferably a dielectric constant of 5 or higher. The ferroelectric
insulator nanoparticles used for the preparation of the first
insulating layer include, but are not limited to, nanoparticles of
barium strontium titanate (BST), PbZr.sub.xTi.sub.1-xO.sub.3 (PZT),
Bi.sub.4Ti.sub.3O.sub.12, BaMgF.sub.4,
SrBi.sub.2(Ta.sub.1-xNb.sub.x).sub.2O.sub.9,
Ba(Zr.sub.1-xTi.sub.x)O.sub.3 (BZT), BaTiO.sub.3, SrTiO.sub.3 or
Bi.sub.4Ti.sub.3O.sub.12. The diameters of the nanoparticles are
not particularly limited, but preferably range from 1 to 100
nm.
In the OTFT of the present invention, the second insulating layer
of the gate insulating film is disposed directly beneath the
organic active layer and made of an insulating organic polymer
compatible with the organic active layer. Like the first layer, the
second layer is formed by the wet process. The organic polymer
suitable for the second insulating layer includes polyvinylphenol,
polymethylmethacryalate, polyacrylate, polyvinylalcohol, or polymer
represented by the following Formula 1: ##STR00001##
[Wherein, R is represented by the following Formula 2:
##STR00002##
(wherein, R.sub.1 is selected from the following groups, in which n
is an integer of 0 to 10: ##STR00003##
R.sub.2 is a photo-alignment group selected from the following (I)
and (II), provided that at least one of R.sub.2 is selected from
(I) when l is 2 or higher: ##STR00004##
R.sub.3 is a hydrogen atom or is selected from among the following
groups, in which X is a hydrogen atom, an alkyl or alkoxy group of
1 to 13 carbon atoms, an aromatic group of 6 to 20 carbon atoms, a
hetero-aromatic group of 4 to 14 carbon atoms having at least one
hetero atom contained in an aromatic ring,
--(OCH.sub.2).sub.pCH.sub.3 (p is an integer of 0 to 12), F or Cl
and m is an integer of 0 to 18: ##STR00005##
k is an integer of 0 to 3 and l is an integer of 1 to 5, provided
that each of R.sub.1 and R.sub.2 can be different when k or l is 2
or higher);
m is a real number of 0.3 to 0.7, and n is a real number of 0.3 to
0.7, provided that the sum of m and n becomes 1; x is a real number
of 0.3 to 0.7, and y is a real number of 0.3 to 0.7, provided that
the sum of x and y becomes 1; and i is a real number of 0 to 1 and
j is a real number of 0 to 1, provided that the sum of i and j
becomes 1].
In the case where a photo-alignment group is introduced to the
insulating organic polymer, like polymer of Formula 1, orientation
of the organic active layer increases so there can be provided
conditions favorable for the formation of the organic active layer
and thus a grain size of the active layer can be larger. As a
result, the transistor characteristics including the charge
mobility between the insulator and the active layer can be
enhanced. The preferable examples of the polymer represented by
Formula 1 include the ones represented by the following Formulas 3
to 6: ##STR00006## ##STR00007##
In the OTFT of the present invention, the wet process that can be
used to prepare the first or the second layers of the gate
insulating film is exemplified by dip coating, spin coating,
printing, spray coating, or roll coating techniques, but is not
limited thereto.
According to the present invention, not only does the multi-layered
gate insulating film have superior insulating properties, but also
the OTFT obtained therefrom shows high charge mobility, low driving
voltage, low threshold voltage and excellent I.sub.on/I.sub.off
value, compared with the OTFT using the single-layer insulator. In
particular, the preparation of the gate insulating film can be
achieved by wet process, such as printing or spin coating, while
the OTFT produced thereby can rival a TFT of inorganic insulating
films prepared by CVD process in its performance.
In the OTFT of the present invention, the organic active layer can
be made of any materials known as an organic semiconductor
including a conducting polymer. Preferably, the organic active
layer is prepared from pentacene, copper phthalocyanine,
polythiophene, polyaniline, polyacetylene, polypyrrole,
polyphenylene vinylene or derivatives thereof, but is not limited
thereto.
In the OTFT of the present invention, materials for the substrate,
the gate electrode and the source/drain electrode can be any known
materials in the art of the thin film transistor. Preferably, the
substrate is made of plastic, glass, quartz, or silicon, and the
gate and source/drain electrodes are made of gold (Au), silver
(Ag), aluminum (Al), nickel (Ni), indium thin oxide (ITO), but are
not limited thereto.
According to the preferable embodiment of the present invention,
the OTFT can be prepared by a process comprising the steps of:
providing the gate electrode disposed on the substrate and forming
a first layer of a high k material, a second layer of an organic
insulating polymer compatible with the organic active layer, the
organic active layer and the source/drain electrode sequentially,
wherein the first and the second layer is disposed through a wet
process such as spin coating, the second layer is positioned
directly beneath the organic active layer and the layering order
between the organic active layer and the source/drain can be
reversed.
Hereinafter, the present invention will be described in more detail
with reference to the following Examples. However, these examples
are provided only for illustrative purposes and are not to be
construed as limiting the scope of the present invention.
PREPARATIVE EXAMPLES 1 TO 4
Preparation of First Insulating Layer of High k Material
Polyvinylbutyral (PVB) and tetrabutyl titanate
(Ti(OC.sub.4H.sub.9).sub.4) were mixed according to a composition
ratio shown in the following Table 1 for each Preparative Example,
and the resulting mixture was dissolved in isopropyl alcohol to
prepare a 10 20 wt % solution. The solution was coated on an
aluminum substrate by spin coating method to form a 2000 .ANG.
thick film, which was then thermally cured at 70.degree. C. for 1
hour and then 150.degree. C. for 30 min, thus yielding a first
insulating layer. An aluminum substrate was placed on the first
insulating layer to manufacture a metal-insulating film-metal
structured capacitor. By use of the capacitor, insulating
properties were measured at 100 kHz. The results are shown in Table
1, below.
TABLE-US-00001 TABLE 1 Preparative PVB Ti (OC.sub.4H.sub.9).sub.4
Ti k (dielectric Ex. No. (wt %) (wt %) (wt %) constant) 1 75 25 40
5.6 2 50 50 66 15 3 30 70 82 27 4 10 90 95 30
From the above table, it can be seen that a dielectric constant can
increase up to 30 by controlling the amount of titanate.
EXAMPLE 1
On a glass substrate having a gate electrode made of aluminum, a
first insulating layer was formed in the same manner as in
Preparative Example 2. A cyclohexanone solution (100 wt %) of a
polymer (S1) represented by Formula 3 was prepared, and spin-coated
on the first insulating layer to form a 5000 .ANG. thick film,
which was then baked at 100.degree. C. for 1 hour in a nitrogen
atmosphere, to prepare a two-layered gate insulating film having a
total thickness of 700 nm. Then, a 700 .ANG. thick pentacene
organic active layer was formed on the gate insulating film by
using an OMBD (Organic Molecular Beam Deposition) process, which is
performed under 2.times.10.sup.-6 torr with a deposition rate of
0.3 .ANG./sec while maintaining a substrate temperature at
80.degree. C. On the active layer thus obtained, a source/drain
electrode was formed by a top contact method using a shadow mask
having a channel length of 100 .mu.m and a channel width of 1 mm,
thereby fabricating an OTFT. For the OTFT thus obtained, a
dielectric constant per unit area (C.sub.0: nF/unit area),
threshold voltage, I.sub.on/I.sub.off value and charge mobility
were measured in accordance with the following procedures. The
results are shown in Table 2 below.
(1) Dielectric Constant Per Unit Area, C.sub.0
A dielectric constant showing dielectric properties was determined
by the following equation: C.sub.0=.di-elect cons./.di-elect
cons..sub.0(A/d)
(wherein, A denotes an area of the device; d denotes a thickness of
a dielectric; and .di-elect cons. and .di-elect cons..sub.0 denotes
a dielectric constant of the dielectric and vacuum,
respectively).
(2) Charge Mobility and Threshold Voltage
The charge mobility was determined from the following current
equation of saturation region. That is, after obtaining a plot of
the square root of the source-drain current [(I.sub.SD).sup.1/2] as
a function of gate voltage (V.sub.G), the charge mobility
(.mu..sub.FET) was calculated from a slope of the plot referring to
the following equations:
.times..times..times..mu..function..times..mu..times..times..times..times-
..times..times..mu..times..times..times..times..mu..times..times..times..t-
imes. ##EQU00001##
(wherein, I.sub.SD denotes a source-drain current; .mu. or
.mu..sub.FET denotes a charge mobility; C.sub.o denotes capacitance
per unit area; W denotes a channel width; L denotes a channel
length; and, V.sub.G and V.sub.T denote a gate voltage and a
threshold voltage, respectively).
The threshold voltage (V.sub.T) was determined from an intersection
point between a V.sub.G axis and an extension line of linear
portion of the plot of (I.sub.SD).sup.1/2 and V.sub.G. The smaller
absolute value of the threshold voltage that are close to 0 means
the smaller consumption in the electric power.
(3) I.sub.on/I.sub.off Value
I.sub.on/I.sub.off value can be determined from a ratio of a
maximum current in the on-state to a minimum current in the
off-state and it satisfies the following equation:
.mu..sigma..times..times..times. ##EQU00002##
(wherein, I.sub.on is a maximum current value; I.sub.off is an
off-state leakage current; .mu. is a charge mobility; .sigma. is a
conductivity of a thin film; q is a charge amount; N.sub.A is a
charge density; t is a thickness of a semiconductor film; C.sub.0
is an oxidation film capacity; and V.sub.D is a drain voltage).
As a dielectric constant of the dielectric film is higher and a
thickness thereof is thinner, I.sub.on/I.sub.off becomes large.
Thus, the kinds and thickness of the dielectric film can be an
important factor determining I.sub.on/I.sub.off value. The
off-state leakage current, I.sub.off is a current flowing in the
off-state and can be determined as a minimum current in the
off-state.
FIG. 2 shows the change of a leakage current while varying the
applied voltage to the OTFT. From FIG. 3 showing the change of
I.sub.SD to V.sub.G, it can be seen that when the gate insulating
film of the present invention was used, a curve was shifted to be
close to 0, which means a lower threshold voltage. Also, from the
plot of (I.sub.SD).sup.1/2 and V.sub.G shown in FIG. 4, it can be
seen that the threshold voltage of the OTFT in the present
invention decreased to 50% or more.
EXAMPLE 2
An OTFT was prepared in the same manner as in Example 1, with the
exception that the first insulating layer was formed using the
composition and the solvent under the conditions same as in
Preparative Example 3. For the OTFT, a dielectric constant per unit
area (C.sub.0: nF/unit area), threshold voltage, I.sub.on/I.sub.off
value, and charge mobility were measured in accordance with the
same procedures as Example 1. The results are shown in Table 2
below.
EXAMPLE 3
An OTFT was prepared in the same manner as in Example 1, with the
exception that a 300 nm-thick first layer and a 400 nm-thick second
layer were adopted. For the OTFT, a dielectric constant per unit
area (C.sub.0: nF/unit area), threshold voltage, I.sub.on/I.sub.off
value, and charge mobility were measured in accordance with the
same procedures as Example 1. The results are shown in Table 2
below.
COMPARATIVE EXAMPLE 1
An OTFT was prepared in the same manner as in Example 1, with the
exception of using a single-layer gate insulating film obtained
from a cyclohexanone solution (10 wt %) of S1, which was then
spin-coated at a thickness of 7000 .ANG. and then baked at
100.degree. C. for 1 hour in a nitrogen atmosphere, instead of the
multi-layered gate insulating film. For the OTFT, a dielectric
constant per unit area (Co: nF/unit area), threshold voltage,
I.sub.on/I.sub.off value, and charge mobility were measured in
accordance with the same procedures as Example 1. The results are
shown in Table 2 below.
EXAMPLE 4
An OTFT was prepared in the same manner as in Example 1, with the
exception of using as the second insulating layer PVP
(polyvinylphenol) film prepared by dissolving PVP in PGMEA
(Propylene Glycol Methyl Ether Acetate) to obtain a 15 wt %
solution, which was then spin-coated at a thickness of 5000 .ANG.
and then baked at 100.degree. C. for 1 hour in a nitrogen
atmosphere. For the OTFT, a dielectric constant per unit area
(C.sub.0: nF/unit area), threshold voltage, I.sub.on/I.sub.off
value, and charge mobility were measured in accordance with the
same procedures as Example 1. The results are shown in Table 2
below.
COMPARATIVE EXAMPLE 2
An OTFT was prepared in the same manner as in Example 1, with the
exception of using a single layer gate insulating film obtained by
dissolving PVP in PGMEA to obtain a 15 wt % solution, which was
then spin-coated at a thickness of 5000 .ANG. and then baked at
100.degree. C. for 1 hour in a nitrogen atmosphere, instead of the
multi-layered gate insulating film. For the OTFT, a dielectric
constant per unit area (C.sub.0: nF/unit area), threshold voltage,
I.sub.on/I.sub.off value, and charge mobility were measured in
accordance with the same procedures as Example 1. The results are
shown in Table 2 below.
TABLE-US-00002 TABLE 2 1.sup.st C.sub.0 Ex. Insulating 2.sup.nd
Insulating (nF/unit Threshold Charge No. Layer Layer area) Voltage
(V) I.sub.on/I.sub.off Mobility 1 PVB:Ti(OC.sub.4H.sub.9).sub.4 S1
(500 nm) 7.0 -11 1.02 .times. 10.sup.4 3 5 50:50(200 nm) 2
PVB:Ti(OC.sub.4H.sub.9).sub.4 S1 (500 nm) 7.9 -9 1.02 .times.
10.sup.4 3 5 30:70(200 nm) 3 PVB:Ti(OC.sub.4H.sub.9).sub.4 S1 (400
nm) 8.6 -7 7.76 .times. 10.sup.3 3 5 50:50(300 nm) 4
PVB:Ti(OC.sub.4H.sub.9).sub.4 PVP (500 nm) 7.3 -13 1.24 .times.
10.sup.5 6 50:50(200 nm) C. 1 -- S1 (700 nm) 5.9 -15 6.67 .times.
10.sup.3 3 5 C. 2 -- PVP (700 nm) 5.5 -17 0.71 .times. 10.sup.5
6
From the above table, it can be seen that the OTFT of the present
invention has high charge mobility, high I.sub.on/I.sub.off, low
driving voltage, and low threshold voltage, with superior
electrical insulating properties. Thus, the OTFT of the present
invention can be effectively applied as a transistor in various
electronic devices.
As described above, according to the present invention, there is
provided an organic thin film transistor comprising a multi-layered
gate insulating film, which exhibits advantageously low driving and
threshold voltages, high charge mobility and high
I.sub.on/I.sub.off. Further, the insulating film of the present
invention can facilitate the formation of the organic active layer
and as well, it can be formed by a wet process, thus simplifying a
preparation process and decreasing preparation costs. The OTFT of
the present invention can be usefully applied in flexible display
fields.
The present invention has been described in an illustrative manner,
and it should be understood that the terminology used is intended
to be in the nature of description rather than of limitation. Many
modifications and variations of the present invention are possible
in light of the above teachings. Therefore, it should be understood
that within the scope of the appended claims, the invention may be
practiced otherwise than as specifically described.
* * * * *